Tactile sensation is paramount to how we experience the physical world around us. Species like mice use their facial whiskers to navigate and assess their environment. Although there is a basic understanding of how brainstem trigeminal neurons, the first central relay for whisker-derived signals, respond to passive tactile stimuli, little is known about their activities and functions during active behaviors. Tactile exploration is an active sensing process, which adapts to behavioral purposes, and responses of trigeminal brainstem neurons are likely adjusted accordingly. Furthermore, brainstem projection neurons contribute to two main ascending pathways, lemniscal and paralemniscal, but the functions of these two distinct pathways remain unclear. Our goal for this high-risk exploratory grant application is to develop a chronic in vivo recording preparation, perform recordings from optogenetically identified lemniscal or paralemniscal projection neurons during active behaviors, and use causal manipulations to determine their functions. Recording and manipulating pathway-specific neurons in vivo, and making accurate behavioral measurements in freely moving animals are both challenging. We will use a combination of technological innovations to overcome these challenges. The outcome of this project will establish a platform for future in-depth dissection of how touch sensory information is integrated, transformed and delivered to different processing streams by trigeminal brainstem to generate perception, invoke emotion and/or inform decisions.